The invention disclosed and claimed herein is generally directed to a method for correcting errors which contaminate, or are contained in, magnetic resonance imaging (MRI) data, and which result from translational motion in a patient or other object of imaging. More particularly, the invention pertains to a correction method of the above type which employs sets of correction data points that are acquired very rapidly so as to be substantially unaffected by the motion. Even more particularly, the invention pertains to a correction method of the above type wherein the correction data are acquired at some of the same data point locations as the imaging data.
In conventional MRI, the scan time typically lasts a few seconds to several minutes. During this time, physiologic motion (e.g., cardiac, respiratory, gastrointestinal, and vascular motion), as well as a patient's gross movements (both voluntary and involuntary) can contaminate the spatially encoded MR signals, causing ghosting and blurring artifacts. Some of these motions, such as cardiac, respiratory and vascular motion, or blood flow, are periodic in nature. Other motions, such as involuntary motion of a patient, or uncontrolled movements of small children, are non-periodic and thus tend to be random or unpredictable.
In a conventional imaging technique such as spin warp, k-space is sampled by a series of lines parallel to the frequency-encoding axis (k.sub.x -axis), with each line corresponding to a unique location along the phase-encoding axis (k.sub.y -axis). Typically, each k.sub.x -line is acquired with a single pulse sequence. The acquisition time lasts only a few milliseconds. Motion during this short acquisition time, known as intra-view motion, is negligible and does not cause substantial image degradation. However, different k-space lines along the phase-encoding direction are acquired by repeating the pulse sequence with different phase-encoding gradients. The time span among the k-space lines can be hundreds of milliseconds or even seconds, making the k-space data along the phase-encoding direction very susceptible to motion. Such motion, referred to as view-to-view motion, can cause serious artifacts in images.
In the past, two principal techniques have been used to correct, or to provide compensation for motion. In one of such techniques, known as gating, data acquisition is synchronized with the motion. Such technique is discussed, for example, by W. J. Rogers, Jr., and E. P. Shapiro in "Effect of RR interval variation on image quality in gated, two-dimensional, Fourier MR imaging", Radiology, vol. 186, pp. 883-887 (1993). However, the gating technique can only be used in connection with motion which is periodic. Moreover, such technique will significantly slow down data acquisition if the periods of successive motion cycles are comparatively long, e.g., on the order of seconds.
In another motion correction technique, known as navigator echo correction, an additional echo is acquired in the same pulse sequence that acquires the k-space data. This echo, referred to as navigator echo, is used to determine the instantaneous position of the object when the sequence is played out, and subsequently to correct the k-space data acquired by the same sequence. In a spin-warp pulse sequence, navigator echo correction doubles the total amount of data acquisition. In fast imaging sequences, such as multi-shot echo planar imaging (EPI) and multi-shot fast spin echo (FSE), navigator echo correction reduces the useable echo train length, which can lead to longer acquisition times. This technique is described, for example, by R. L. Ehman and J. P. Felmlee, Radiology, vol. 173, pp. 255-263 (1989), and by Z. W. Fu, et al., Magn. Reson. Med., vol. 34, pp. 746-753 (1995).
In view of the above disadvantages of the prior art, it would clearly be desirable to correct errors in MR imaging resulting from view-to-view motion, wherein the technique used for correction does not significantly increase data acquisition, and applies to motion which may be either periodic or non-periodic.